A .TXT file I had from 2009, probably from my late great Watsonseblog. If it had a pic attached, I’d like to add it, but I’m unable to determine what picture.
Joule Thief using Air Core Inductor – Feb 13, 2009
I saw Quantsuff’s Valentine’s Day Flashing Heart on Instructables and it inspired me to experiment with some air core inductors. The Joule Thief doesn’t have to have an inductor using a toroid, but the toroid helps a lot, as I’ll explain below. The coil has to have inductance so it can store a charge in the magnetic field and then dump it into the LED when the transistor turns off. You can use a coil of wire with an air core.
With a toroid core, the wire is much shorter and the resistance lower, so it will have less losses in the wire. And the coil will be much smaller, small enough to fit into the base of a flashlight bulb. You can’t do that with a large air core coil. But if you have space for an air core coil, you can make a Joule Thief work with one.
Let’s say you use a toroid, and wind it with a foot of 24 gauge wire. It might have an inductance of 100 to 300 microhenrys, which is great for a JT. That wire will have a DC resistance of 25.7 milliohms or .0257 ohm. That’s so low it will have very little loss.
In the instructable, QS said to use 6 feet of wire on a spray can lid. I wound six feet of 24 gauge telephone wire on a spray can lid 2-1/4 inches in diameter. I then slid the coil off and taped up the windings. The winding had about 8.5 microhenrys inductance, which is quite low for a Joule Thief (typically 100 microhenrys or so), and much lower than what QS said it should be (22 microhenrys). The telephone wire has two wires twisted into a pair, one is white with blue stripes, the other is blue with white stripes. You can use finer wire for the feedback winding, but the primary winding should be heavy wire to minimize losses.
The 6 feet of 24 gauge wire had a resistance of .15 ohm. That doesn’t seem like much, but when you are working with only 1 volt and several hundred milliamps, that resistance can cause losses. I calculated that it could drop about 1/20 volt, and have more than a milliamp of loss. That’s more than 1 percent of the total LED power.
The coil drew about a hundred milliamps and ran at over 300kHz, which is too high for this circuit. I didn’t think that a coil of 8.5 microhenrys was enough inductance. It should be 22 microhenrys, as QS stated. So I wound another coil with ten feet of the same wire. This one came out much better, it measured 25 uH. When I taped it up, it was a bit higher, closer to 30 uH. I connected this to the Joule Thief circuit and it drew 100 milliamps from the 1.5V supply and ran at 139 kHz. This was much closer to what a Joule Thief should be.
But the DC resistance of ten feet of 24 gauge wire was over a quarter ohm, 0.257 ohm to be more specific. The loss was more than twice that of the first coil I made. This is not a good thing. What I really should do is use heavier wire, or put another winding in parallel with the primary winding.
Update Sep 13, 2009
I have since wound several more air core coils, some with 24 gauge insulated telephone wire, some with enameled wire, and some with both. I have found that because of the plastic insulation on the telephone wire, the turns of the winding can’t get close together and this reduces the inductance and makes the coil larger, allowing fewer turns in the same space. This is not a big drawback if the coil is wound on a core of, say, ferrite, since the permeability of the core increases the inductance and less wire is needed. But when the coil is wound on an air core, the permeability is low, actually unity, and the wire has to be longer to get the same inductance. Thus for best air core performance, it is best to use enameled wire with thin insulation.
I pursued another way of winding a coil. I used 6 feet of four pair telephone wire, which when each pair was connected to the next, was equivalent to a single pair of 24 feet wound into a coil. The wire is cheap and easy to find. But connecting the pair to one another and taping them up takes time and is tedious. This can be automated somewhat with telephone type blocks. Another disadvantage is the insulated jacket puts more space between the wires, lowering the inductance somewhat. And as in the above case, this increases the resistance per winding. But I did manage to get a Joule thief working with this coil. I used my Supercharged JT circuit.